The usual printed wiring boards, ceramic substrates, printed circuits and the like include a multiplicity of conductive paths or lines of etch which are connected selectively and which lie in relatively complex patterns on a non-conductive rigid or flexible substrate. Prior to connecting the various IC chips, resistors, capacitors, etc. to the circuit, it is desirable to test the electrical conductors of the circuit so that circuit faults such as opens and shorts can be discovered and corrected before those circuit components are added to the board or circuit.
It has long been the practice to test such circuits by placing the circuit in a fixture and bringing "bed of nails" test probes into contact with the various circuit conductors or lines of etch. Once in contact, the continuity between various probes is measured using well known electrical instrumentation. However, as the packing densities of such circuits have increased over the years, the conductor widths and pad sizes have decreased commensurately. Resultantly, it has become more and more difficult to use such mechanical means because of the need for finer and finer probes and more accurate motion systems for positioning the probes relative to the circuit being tested. In addition, there is a greater propensity for damaging finer conductors and probes during the test.
To meet these more stringent testing requirements, various techniques have been developed to reduce or eliminate entirely physical contact with the circuits during the performing of such tests. One known technique uses an electron beam. One end of a conductor in the circuit is bombarded by an electron beam and brought to a predetermined potential by secondary electron emission so that between the two ends of the conductor, there is a potential difference resulting in current flow along the conductor provided there is no break in the conductor. The prior art is replete with testing systems using this principle which provide clear discrimination between uninterrupted and interrupted conductors as well as detection of shorted conductors in printed wiring boards, circuits, VLSI packaging substrates and other microcircuitry. However, contactless testing using an electron beam does have some disadvantages. Some such systems require individually controlled electron beams which must simultaneously address both ends of the conductor under test. Some electron beam test systems require relatively complex masks which make difficult the loading and unloading of specimens.
There is also a known non-contact method of testing electrical conductors in which a single mechanical probe stimulates a conductor of a circuit placed in a low pressure inert atmosphere. This stimulation causes the conductor and all the lines and runs connected electrically to it to glow. This glow is then observed using a scanning photometer. In this way, an entire network can be checked with the use of a single mechanical probe thereby minimizing the contact between the probe and the circuit under test. This prior testing technique is disadvantaged in that it does require at least some physical contact between the test instrument and the circuit under test.
There is also a known prior fault testing technique using a laser for achieving an electrical connection between a probe and a circuit under test without there being any physical contact between the probe and the circuit conductors. In this arrangement, a laser produces a plasma that electrically connects the probe positioned above the circuit under test to the underlying circuit conductor. However, a mechanical connection is still needed to electrically connect the plasma "probe" to the necessary instrumentation to determine whether or not there is a circuit fault.